The present invention relates to capacitors having increased charge storage and a method for their production. More specifically, the present invention relates to electrochemical double layer capacitors in which axi-symmetric, high surface area electrode growth is produced and nanoscale electrolyte confinement tenability allows for electrolyte generality and enhanced capacitance.
Electrochemical-based capacitive energy storage is based on charge adsorbed within the electric double layer to store electrical energy. [Kotz and Carlen; Principles and applications of electrochemical capacitors. Electrochimica Acta, 2000, (45), 2483-2498; Simon and Gogotsi; Materials for electrochemical capacitors. Nat. Mater., 2008, (7), 845-854] Due to the high specific surface area of nanomaterials and the mechanism of electrochemical double layer capacitor (EDLC) charge storage, many believe that EDLCs can bridge the gap between batteries and capacitors with respect to power and energy density [Simon and Gogotsi; Materials for electrochemical capacitors. Nat. Mater., 2008, (7), 845-854; Conway; Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. 1999,]. DLC electrode materials, which primarily determine the extent of charge storage and partially determine the rate of charge transport/transfer, are commonly metal oxide- or carbon-based. [Kotz and Carlen; Principles and applications of electrochemical capacitors. Electrochimica Acta. 2000, (45), 2483-2498; Jayalakshmi and Balasubramanian; Simple Capacitors to Supercapacitors—An Overview. Int. J. Electrochem. Soc., 2008, (3), 1196-1217]. Most commercial EDLCs use activated carbon electrodes and it is well understood that the pore size in the activated carbon (AC) highly influences specific capacitance. [Huang, Sumpter and Meunier; A Universal Model for Nanoporous Carbon Supercapacitors Applicable to Diverse Pore Regimes, Carbon Materials, and Electrolytes. Eur. Chem. J., 2008, (14), 6614-6626]. The surface area of AC can be high (2600 m2/g), but much of this surface area is inaccessible to the electrolyte and the pore size is distributed over a large range. Therefore, the charge/discharge rate is mass transfer limited which leads to low specific energy densities and the pore size distribution does not optimize charge adsorption for a given electrolyte. [Ervin; Carbon Nanotube and Graphene-Based Supercapacitors: Rationale, Status, and Prospects. 2010, (ARL-TR-5283)].
Many other forms of carbon including fibers, various nanostructures, and graphene have also been used as a component of EDLC electrodes. Single- and multi-walled carbon nanotubes (SW- and MW-NTs) are of particular interest due to their material properties, radial symmetry and high surface area. SWNTs offer the second highest electrolyte accessible surface area of all carbon matrices (graphene is highest), but are more desirable than graphene as they can be controllably oriented; lending to faster electrolyte percolation and facile electrochemical kinetics. [Ervin; Carbon Nanotube and Graphene-Based Supercapacitors: Rationale, Status, and Prospects. 2010, (ARL-TR-5283; Peigney, Laurent, Flahaut, Bacsa and Rousset; Specific surface area of carbon nanotubes and bundles of carbon nanotubes. Carbon, 2001,(39), 507-514; Stoller, Park, Zhu, An and Ruoff; Graphene-Based Ultracapacitors. Nano Letters, 2008, (8), 3498-3502.] However, current fabrication methods fail to produce 100% metallic SWNTs. Two thirds of the SWNTs produced are semiconducting, thus they are not ideal for use as an electrode. Bulk CVD production of MWNTs yields high-strength and -thermal conductivity structures that are nearly 100% electrically conducting. The CVD method also produces MWNTs that can be oriented in arrays, thus maximizing kinetics and electrolyte percolation. In the limit of low diameter (<10 nm), MWNTs are better suited as EDLC electrode materials because their number density approaches that of SWNTs and roughly 67% more of the array is electrically conducting.
An object of our invention is to provide a method to precisely tune the degree of electrolyte confinement (pore size) around an ultra-high surface area electrode. Yet another object of our invention is to produce an electrode-electrolyte system exhibiting enhanced charge storage with applications in electrochemical energy storage systems, such as electrochemical double layer capacitors, solid state capacitors, and batteries. We have achieved this with a recognition of the surprising advantages achievable through a combination of templated fabrication of high density aligned MWNTs and confinement of the electrolyte around each electrode, enhancing charge storage in the system.
In a currently contemplated embodiment of our invention, anodized aluminum oxide (AAO) is used to template the growth of high density arrays of MWNTs, as well as to enhance charge adsorption beyond that which is expected due to adsorbed charge within the Helmholtz planes of the double layer. As is known, AAO makes an ideal template for the fabrication of nanotubes with controlled orientation and dimensions due to the scalable and reproducible electrochemical self-assembly of ordered pores within aluminum during oxidation. [Parkhutik and Shershulsky; Theoretical Modeling of Porous Oxide-Growth on Aluminum. J. Phys. D. 1992, (25), 1258-1263;: Ahn, Sohn, Kim, Shim, Kim and Seong; Electrochemical capacitors fabricated with carbon nanotubes grown within the pores of anodized aluminum oxide templates. Electrochem. Comm., 2006, (8), 513-516; Hill, Haller and Ziegler; Direct Fabrication of High-Aspect Ratio Anodic Aluminum Oxide with Continuous Pores on Conductive Glass. J. Electrochem. Soc., 2010, (158), E1-E7] The additional charge storage from the diffuse domain of the double layer arises from anisotropic and tunable confinement of the electrolyte region around each MWNT by selectively and partially etching the AAO template. Templated-growth of the MWNTs allows for structures with diameters below 10 nm, lengths exceeding 1 cm, and MWNT array densities exceeding 1011 cm−2 to be achieved. Furthermore, Raman spectroscopy indicates that high quality MWNTs are obtained from the templated CVD fabrication, and template-selective etching does not affect the MWNT structure. As high as an 8-fold increase in capacitance is observed when the template is partially etched compared to a completely etched MWNT array in an acetonitrile solvent system containing tetraethylammonium tetraflouroborate. This capacitance enhancement is expected with different electrolytes at different partial etching times (i.e. different electrolyte confinement size as defined by the MWNT surface to AAO pore wall distance). A specific capacitance in excess of 315 F/g-carbon is demonstrated using 10 nm diameter, 10 μm long MWNTs that are arrayed at a density of 4.5×1011 cm−2.